Platelets prevent hemorrhage by a secretory process controlled through an intracellular chain of biochemical events similar to that in all other secretory cells. Tyrosine kinase (TK) activity, which causes tyrosine phosphorylation (TP) of specific cellular proteins is temporally associated with secretion by platelets and other cells, as well as with cellular responses such as growth, contact inhibition, and malignant transformation. The role of TP in these processes is not known. Last year, we found increased platelet cytoplasmic calcium that accompanies secretion promotes TP of a 130 kD major cytoplasmic protein, while a homeostatic level of calcium in storage compartments promotes tyrosine dephosphorylation of this protein. (J Biol Chem 226:16911-16916, 1991). This year, we identified the 130 kD platelet protein as vinculin by its reaction with both monoclonal anti-phosphotyrosine and anti-vinculin on Western blot, by affinity chromatography isolation with both antibodies, and the isoelectric focusing of a single protein with a pI for vinculin that reacted with anti- vinculin and anti-phosphotyrosine. Since TP of vinculin is under control of cytosolic and stored calcium and vinculin is known to be linked via alpha-actinin to platelet glycoprotein IIb/IIIa which mediates calcium influx, vinculin may be involved in regulation of platelet membrane calcium channels.